Shape memory articles (SMAs), comprised, for instance, substantially of NiTinol alloy, are used in many surgical applications, including use as staples for re-attaching tissue or bone. Usually, external heat is applied to the shape memory article in order to transition it from a first shape in a martensitic, softer, morphology to a second shape in an austentitic, stiffer, morphology.
When a patient suffers an injury in which tissue or bone must be reapproximated, reattached, or fused, the injury often must be repaired by surgically securing the tissue or bone together with internal fixation devices such as plates, screws, pins, or staples. These devices are often rigid and have geometric features that enable them to reapproximate, reattach, or fuse tissues. Examples of these features include threads, grooves, overall shape of the device, and other features that provide attachment or support. Any undesired deformation of these devices could lead to increased amounts of strain and ultimate failure of the device.
Since the late 1980's, NiTinol, a Nickel-Titanium alloy, has been increasingly utilized in a variety of medical devices and, in some cases, has become one of the materials of choice for many designers and engineers. From surgical devices to endoluminal stents and other prostheses, the thermo-mechanical characteristics of the material and its biocompatibility have allowed its use across many medical and surgical specialties both for diagnostic and therapeutic applications.
The shape memory effect results from a reversible crystalline phase change known as martensitic transformation. Shape memory alloys can display various types of shape memory. The type of shape memory that has probably found the most use in commercial applications is commonly referred to as one-way shape memory. In one-way shape memory, an article formed of a shape memory alloy in an original shape can be substantially plastically deformed into a shape while it is in the soft, martensitic phase and it will remain in that shape, (hereinafter the deformed shape). Then, upon heating above a first temperature, the material returns to its original (prior to deformation) shape while transitioning from the soft, martensitic phase to a much stiffer austentitic phase. It should be noted that, while the article is much stiffer in the austentitic phase, it usually is still somewhat deformable, but primarily elastically, as opposed to plastically, deformable. Upon cooling below a second temperature that is below the first temperature, the material transitions back to the softer, martensitic phase, but maintains the shape it took during the transformation to the austentitic phase (i.e., its original shape) until it is acted upon by an external force or stress. Because the material is less stiff (i.e., more pliable) in its martensitic phase, it is much easier to bend (back to the deformed shape or any other shape) and it will maintain that new shape up to and until it is heated once more above its transformation temperature.
The strength and transition temperatures of SMAs can be greatly varied by changing the exact composition of the alloy and/or the thermal history of the article.
The use of shape memory staples in surgical skeletal repair enables a staple to be installed in bone or tissue in one shape while in its martensitic phase and then be heated to cause it to transition to the much stiffer austentitic phase while shifting to another shape that, for instance, draws the tissue or bone closer together. Many medical applications use SMAs having a transition temperature for complete martensitic to austentitic transformation of about 55° C. However, other medical applications utilize alloys having a complete transition temperature at about human body temperature of 37° C.
While metallic staples have long been used for static fixation, the use of shape memory alloys (SMAs) in staples and their attendant ability to apply dynamic continuous compression is a major advancement in tissue and bone uniting that potentially improves the healing process in connection with the repair, fusing, and remodeling of damaged tissue. These SMA staples are smaller and less bulky than other fixation devices, such as plates, screws, and nails. They permit smaller incisions, which cause less trauma and scarring and lead to faster post-operative recovery. Also, since fewer holes need to be drilled and no screws are needed, more rapid surgical procedures are possible.
The shape memory properties described hereinabove are sometimes referred to as superelasticity, particularly when the transition from martensitic phase to austentitic phase occurs at lower temperatures, such as room temperature or below. The terminology is not consistent in the art. In this specification, we shall simply use the term shape memory generically as encompassing superelasticity.
FIG. 1 is a graph showing a dynamic scanning calorimetry (DSC) for one particular NiTinol composition. DSC is useful for determining the temperatures at which various substances undergo phase changes. In the case of NiTinol or other SMA articles, DSC is utilized to understand the temperatures required for transitioning from the martensitic phase to the austentitic phase and back again. DSC measures the heat flow necessary to maintain the article at a certain temperature. The bottom portion of the scan represents the state of the article at −50° C. as it is subjected to increasing temperature over time. This graph shows a stable structure (martensitic morphology) during temperatures up to an austentitic start temperature (As) of approximately 29° C., where phase transformation to the austentitic phase theoretically begins. As demonstrated by this scan and the change in heat flow, the metal is fully transformed into its stiff, austentitic phase at the austentitic finish temperature (Af) of approximately 50° C. The top portion of this scan represents cooling of the austentitic NiTinol article starting at 100° C. Note that the martensitic phase recovery theoretically begins at the martensitic start temperature (Ms) of approximately 19° C. and is complete at the martensitic finish temperature of approximately 0° C. This is only an example of one form of NiTinol shape memory alloy. Other transition temperatures are achievable with different chemical compositions and thermo-mechanical treatments.
Using the exemplary material above, one can see that the device is geometrically stable in its martensitic phase up to room temperature, can be transformed to an austentitic phase via heating it to around 55° C. and that it stays in a stable austentitic phase down to temperatures well below body temperature. This is very advantageous in surgical applications as devices, such as orthopaedic staples, can be programmed during manufacture with a clinical utility shape in the austentitic phase (the shape that it will take after heating during a surgical procedure) and then be deformed during manufacturing to an operable configuration in its martensitic phase (the shape in which it will be delivered to the surgeon for insertion into the body prior to heating).
Orthopaedic NiTinol staples have been available clinically in the US for approximately ten years. The manufacturers of these devices are using various instruments and power sources for heating the staples in order to effect the transformation to the austentitic phase in vivo. Tissue cautery and coagulation devices typically are available in an operating theater and are commonly used to provide heat to shape memory articles.
Shape memory materials typically have a temperature range of about 20° C. over which they make the transformation from the martensitic phase to the austentitic phase. Thus, for instance, a shape memory article designed to complete its transformation to the austentitic phase at body temperature, i.e., about 37° C., will begin transitioning at temperatures as low as 17° C., or at approximately room temperature.
Thus, shape memory articles, particularly ones designed for body temperature activation often are exposed to temperatures higher than the temperature at which they start the phase change from martensitic to austentitic phase prior to surgery, such as during transportation. Accordingly, shape memory articles often are packaged in the manufacturing plant in a constraining device that prevents them from changing shape until released from the constraining device.
Once a shape memory article has transformed to its austentitic phase, it can be transformed back to martensitic by exposing the shape memory article to a much lower temperature. In the example above, such a transition temperature back to the martensitic phase would occur at or below (minus) 15° C. thus, immediately prior to surgery, shape memory articles commonly are frozen to return them as fully as possible to their original martensitic phase and delivered to the operating room in a frozen or other cold state, such as in a cooler filled with ice.
Using a shape memory surgical staple as an example, a surgeon typically might remove the staple from its packaging and constraining device while in its martensitic phase essentially at the time it is needed for implantation. A surgeon typically might grasp the backspan of the staple with a clamp and pull it out of the constraining device. The surgeon might have an extremely small window of time in which to implant the staple into the patient, e.g., into pre-bored holes in a bone, because the staple may start its transformation from the martensitic phase to the austentitic phase almost immediately upon removal from the constraining device. Particularly, operating rooms are commonly maintained at about the austentitic phase transition starting temperature for body temperature activated shape memory articles.
Even if the operating room is colder than the activation temperature, the surgeon may have to expose the staple to body temperature for a period of time before while he is locating the holes within which the legs of the staple must be inserted, which also could cause the staple to start deforming before it is in the implantation position.
This can be a significant problem during surgery insofar as, once the staple or other shape memory article begins deforming, then its legs may not match up with the pre-bored holes into which they are to be inserted. In such situations, typically, the surgeon would have to discard the staple and start over with a new staple and move much more quickly.